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Researchers are beginning to understand why lasting weight loss is so hard. They believe it has to do with damage to the part of the brain that's involved in weight control.
If obese people stop overeating, switch to a healthful diet and start exercising, they lose weight. However, they may quickly gain it back again. The reason, say researchers, is not a lack of willpower but injury to brain cells, or neurons, in the hypothalamus - a structure deep in the brain that helps control a number of body functions including appetite and weight.

The notion that brain damage might play a role in body weight is not a new one, according to Michael Schwartz of the Diabetes and Obesity Center at the University of Washington in Seattle.

Scientists have known for about five years that the hypothalamus of overweight animals - including humans - displays inflammation, a typical reaction to injury.

But researchers, led by Schwartz, wanted to determine the role hypothalamus injury plays in obesity, and they had several questions they wanted to answer:

"Is it simply a consequence of becoming obese or does it occur before the obesity occurs?" says Schwartz. "And what could be driving that response? What is causing the inflammatory response and could that have something to do with the obesity itself?"

Schwartz's team put laboratory mice and rats on a high-fat diet to make them gain weight. When they looked for evidence of inflammation in their brains, they made a startling discovery.

"We were frankly shocked to realize the inflammation became apparent within 24 hours of the switch in diet," says Schwartz.

The researchers also saw evidence of a strong and rapid neuro-protective response, as cells were activated to repair the damaged neurons. But the animals were kept on the high-fat diet for nine months and the inflammation eventually returned.

The key message, according to Schwartz, is that it's not the fault of people who try and fail repeatedly to lose weight through dieting.

"Our data would point to a more structural, biological basis for why it is difficult to keep weight off," he says. "It has to do with damage to the brain area that is responsible for controlling body weight."

Schwartz says that also explains why drugs are not effective at helping people achieve lasting weight loss. The compounds currently available do not target what appears to be the underlying cause of obesity.